The present invention relates to a process and apparatus for exhanging heat between fluids. One fluid, called the working fluid, is maintained at conditions of pressure and temperature where thermal heat exchanges lead to the evaporation or condensation of a portion of the working fluid mass. More specifically, the present invention concerns heat pump apparatus.
Heat pumps have been known for a long time, but the energy output and cost have not yet reached an acceptable level.
One reason for this lack of general acceptance is that there is no ideal working fluid which is well suited for the operating conditions under which heat pumps are most likely needed: e.g., a fluid in which the working temperatures are on the same order as environmental temperatures. Freon, water and ammonia are examples of commonly used working fluids. Freons work well due to their temperature of condensation at moderate pressure; however, the Freons are expensive, cannot be used in contact with grease or similar lubricants and cannot be released or diffused into the atmosphere. Using water at ordinary temperatures, heat pump installations must have an unacceptably large volume as a result of the corresponding operating low pressures. Ammonia has suitable working pressures in the area of a particular interest, has good heat exchange characteristics, and is far less expensive than the Freons, but ammonia is both corrosive and toxic.
Existing heat pump apparatus is also of importance in evaluating the state of the prior art. More particularly, the presently available heat pumps have mediocre heat exchange coefficients which necessitate comparatively voluminous heat exchangers. A natural consequence of large heat exchangers is the need for large masses of working fluid. This need for a large mass of working fluid is a substantial restriction since the working fluids are either costly, as in the case of Freon, or are dangerous, as in the case of ammonia.
One way of improving existing heat pumps is to improve their thermodynamic efficiency so as to obtain better transfer for a given quantity of working fluid.
An example of this approach is illustrated in a Patent No. CH 305,668 in which evaporation of the working fluid is done in stages, successive stages having decreasing or lower pressures and temperatures. The working fluid is extracted in its vapor stage at each stage and forwarded to a corresponding compressor where it reaches the vapor generated by a lower pressure stage that has already been compressed in another compressor stage. This concept has not achieved widespread acceptance. One may speculate that the reason is that either Freon or water is needed as a working fluid. In addition, the improvement in thermodynamic efficiency was limited since the vapor which eventually exhausts from the compressor is not at saturated thermodynamic conditions.
Another patent, French Pat. No. 7614965, published under No. 2,352,247, presents a more refined solution to the problem of improving the thermodynamic efficiency. In this patent, there are a plurality of modular stages in each of which evaporation of the working fluid takes place in stages. A compressed vapor coming from an adjoining modular stage at lower pressure is placed in contact with the liquid phase at a stage of the compressor so as to bring the vapor back to its saturated thermodynamic conditions. This process describes a zig-zag path on an entropy diagram which path represents the successive states of the vapor phase and always remains in proximity to the saturated vapor curve. With such a process, a variable energy is obtained which depends on the nature of the working fluid and its operating conditions. The process, however, is particularly interesting in the case of ammonia. The advantage of this process is considerably inhanced by the fact that the condensation of the working fluid in the high temperature area of a heat pump is also taking place in modular stages similar to those modular stages where the evaporation is occurring. In that patent, the heat exchangers themselves are not object of a specific concern. The heat exchangers are partially filled by the liquid phase of the working fluid. Tubes carrying the second heat exchanging fluid traverse the working fluid reservoir and attain heat exchange relationship with the working fluid while in the liquid phase or in the superheated vapor phase.
An arrangement such as that in the French patent requires the use of a large mass of working fluid which, as mentioned above, is undesirable especially in the case of ammonia. Moreover, there is no guarantee that the heat exchange coefficients would be favorable.
Accordingly, it is apparent that the need continues to exist for a heat exchanging process and apparatus which overcomes the defects of the types discussed above.